1. Field of the Invention
The present invention relates to a conductivity modulation type MISFET and a control circuit thereof and more particularly, a conductivity modulation type MISFET provided with a parasitic transistor control electrode for controlling its internal parasitic transistor, a control circuit thereof.
2. Description of the Prior Art
A conductivity modulation type MISFET such as insulation gate type bipolar transistor and IGBT incorporating a bipolar transistor despite of having a similar construction to a power MOSFET, is a voltage control type device and featured with a low ON voltage. A typical construction is shown in FIG. 1. In FIG. 1, a conductivity modulation type MOSFET 41 comprises a p.sup.+ -type semiconductor board 42 which is a drain area, an n type buffer layer 43 formed on an obverse surface of the board 42, an n.sup.- -type conductivity modulation layer 44 which isepitaxially formed on the obverse surface, a p-type channel diffusion area 47 which is diffusion-formed with a polysilicon gate 46, which is formed on a silicon oxidized film 45 on the obverse surface, as a mask, and an n.sup.+ -type source diffusion area 48 on the obverse surface. In this case, a parasitic transistor of an n-p-n construction comprises an n.sup.+ -type source diffusion area 48, a p type channel diffusion area 47 and an n.sup.- -type conductivity modulation layer 44 (n-type buffer layer 43). Therefore, when a large current is supplied to the conductivity modulation type MOSFET 41, the parasitic transistor, that is, a parasitic thyristor formed by an n.sup.+ -type source diffusion area 48, a p-type channel diffusion area 47, an n.sup.- -type conductivity modulation layer 44 (n-type buffer layer 43) and a p.sup.+ -type semiconductor board 42 becomes ON (latch-up phenomenon) due to a voltage drop in the p-type channel diffusion area 47 just below the n.sup.+ -type source diffusion area 48 and a turn-off control of the conductivity modulation type MOSFET 41 is impossible. By forming the p.sup.+ -type difuusion area 49 and a source electrode 50, which is resistance-connected to this p typechannel diffusion area 47 and the n.sup.+ -type source diffusion area 48, in the p-type channel difffusion area 49, the voltage drop is controlled to prevent the latch-up phenomenon. A drain electrode is conductively connected to the p.sup.+ -type semiconductor board 42 and a gate electrode 52 is conductive by connected to the polysilicon gate 46.
In the conductivity modulation type MOSFET 41 with the construction as described above, when a positive potential is applied to the gate electrode 52 under the condition that the source electrode 50 is grounded and the drain electrode 51 is forced to have a positive potential, an inversion layer is formed on the obverse surface 53 of the p-type channel diffusion area 47 opposing to the polysilicon gate 46 through the silicon oxidized film 45 and electrons are injected into the n.sup.- -type conductivity modulation layer 44 and, at the same time, holes are injected from the p.sup.+ -type semiconductor board 42, through this inversion layer. As a result, the n.sup. -type conductivity modulation layer 44 is conductivity modulated and an ON resistance reduces. For example, when a current shown with a solid line 61 in FIG. 2 is supplied to the conductivity modulation type MOSFET 41, a large transient ON voltage Vp is produced from the conductivity modulation type MOSFET 41 at a time t.sub.11 when the n.sup.- -type conductivity modulation layer 44 is not yet fully donductiveity modulated as the change of the ON voltage is shown with a solid line 62, then the n.sup.- -type conductivity modulation layer 44 is conductivity-modulated and the ON voltage reduces in time t.sub.12. This low ON voltage is a feature of the conductivity modulation type MOSFET 41.
Since the operation frequency in the conventional circuit construction is several kHz to several 10 kHz, such transient ON voltage Vp has not been a problem and the turn-off characteristics have mainly been improved by employing an anode-short construction. However, if the operating temperature is higher than the above, the turn-off operation is carried out before the n.sup.- -type conductivity modulation layer 44 is conductivity-modulated and therefore the conductivity modulation type MOSFET 41 does not exhibit a low ON voltage which should be an advantage and, on the contrary, noise and increase of loss caused by to a transient ON voltage Vp have been problems.